Geochemical variability as an indicator for large magnitude eruptions in volcanic arcs

Caldera-forming eruptions have the potential to induce drastic socioeconomic change. However, the criteria to identify volcanoes capable of producing large magnitude eruptions in the future are not well constrained. Here we compile and analyse data, revealing that volcanoes which have produced catastrophic caldera-forming eruptions in the past, show larger ranges of erupted magma geochemistry compared to those that have not. This suggests geochemical variability is related to the size of magmatic systems. Using heat transfer simulations, we show that differences in magma flux result in a dependency between chemical diversity and magma volume that is consistent with these observations. We conclude that compositional spread should be included in the catalogue of criteria to identify volcanoes with greater probability of producing future large eruptions. Importantly, this allows to identify stratovolcanoes with caldera-like geochemical signatures, which have not yet been recognized as systems with greater likelihood of producing large magnitude eruptions.

Major element oxides plotted versus SiO2 in weight percent in the compiled dataset. Note that some major element oxides show a large variability over a restricted range of SiO2 contents (e.g. MgO, TiO2), while others show more uniform behaviour (e.g. CaO, FeOt). Fig. S2: Median major element oxides in volcanic bulk rock analyses plotted versus the 95 th percentile range of the respective oxide in weight percent. Volcanic systems that have produced caldera-forming eruptions in the past (orange symbols) are best separated from systems that have not produced such eruptions yet (blue, grey colours) in major element oxides that show relatively uniform variation in igneous differentiation sequences. Symbol size reflects the total lifespan for each of the volcanic systems. Fig. S3: Histograms of bulk-rock SiO2 contents (wt.%) for caldera volcanoes. Grey bars show volcanic rocks of the pre-caldera stage before the first caldera-forming eruption of the system, while orange bars indicate the compositional diversity of syn-and post-caldera stages.

NOTES ON PRE-/ POST-CALDERA DATA COMPILATION
Acoculco. The Acoculco caldera complex in the Eastern Trans Mexican Volcanic Belt was formed around 2.7 Ma by an andesitic ignimbrite. Pre-caldera volcanism (3.0-3.9 Ma) associated with the complex is largely unconstrained. Available bulk-rock compositions of this phase (n=2) are andesitic to dacitic (Avellán et al. 2020).
Aso. The multi-cyclic Asosan caldera in Kyushu (Japan) formed 266±14 ka ago by the evacuation of the Aso-1 ignimbrite. Pre-caldera volcanism in the Aso area has been extensively studied in the caldera walls through radiometric dating, petrography, and whole rock geochemistry (Miyoshi et al. 2009). The pre-caldera eruptive products are mostly younger than 1 Ma and comprise an age range between 2.2 and 0.43 Ma. Like the post-caldera rocks, bulkrock composition of this early phase span the entire range from basaltic to rhyolitic.
Baker-Kulshan. The Kulshan caldera was formed by an ignimbrite around 1.15 Ma ago with minimum volume of 50 km 3 that is only preserved as intra-caldera deposit likely due to extensive glacial erosion in the area. Pre-caldera rocks have been constrained to range from andesite to rhyodacite, while basaltic rocks have to date only be described in the post-caldera phase, which includes Mt. Baker volcano (Hildreth et al. 2003).
Batur. Collapse of the first Batur caldera on Bali (Indonesia) is associated with the 29.3 ka dacitic Ubud Ignimbrite. While the post-caldera petrology and geochemistry of the Batur volcanic complex has been studied in detail, to our knowledge only few whole-rock compositions are available for the pre-caldera phase (n = 2; Nicholls 2004, 2005).
Emmons Lake. The Emmons Lake Volcanic Center (Aleutian arc) is comprised of multiple nested calderas that formed between 27 and 420 ka, producing extensive ashflow tuff deposits (Mangan et al. 2009). While much geochemical work has been carried out on the younger eruptive products, the compositional spread of the pre-caldera episode is unknown.
Gorely. Caldera-forming eruptions at Gorely volcano on the Kamchatka peninsula (Russia) took place between 361 and 38 ka. The pre-caldera stage of the volcano (700-361 ka), termed 'Old Gorely' has been studied in some detail and spans, like the post-caldera stage, the entire range of geochemical compositions from basalt to rhyolite (Seligman et al. 2014;Gavrilenko et al. 2016).
Laguna del Maule. Two Pleistocene ignimbrites have been described and dated in the Laguna del Maule area: the 1.5 Ma Sin Puerto dacite and the 990 ka Bobadilla rhyodacite (Hildreth et al. 2010;Andersen et al. 2017). However, while there is geological evidence that at least the latter of these eruptions resulted in caldera formation, the geochemical diversity of pre-caldera volcanic rocks at Laguna del Maule is at the time of writing unknown.

Los Azufres.
A remnant wall of the Los Azufres caldera has been described in the Santa Inés range on the northern edge of the volcanic field but may pre-date the currently active volcano. If Los Azufres represents a resurgent caldera or a different structure remains debated (Arce et al. 2012). An analysis of pre-and post-caldera volcano geochemistry is at present not possible.
Los Humeros. The Los Humeros caldera (Trans Mexican Volcanic Belt) was formed by the Xaltipan ignimbrite around 164 ka based on recent Ar-geochronology. Pre-caldera volcanism is preserved just outside the caldera rim as rhyolitic domes to the west and north (map units Qr3 and Qr4age <700 ka), as well as basaltic centres dated around 190 ka to the south and north of the caldera structure (map units Qb2). Mafic and silicic pre-caldera eruptions took place in similar distance to the later caldera fault. The Pliocene basaltic to andesitic Teziutlán lava flows (1.46 -2.61 Ma) were not included in the compilation of pre-caldera rocks at Los Humeros but are not unlikely to be part of the same magmatic system given typical lifetimes of caldera volcanoes of several Ma, the proximity of surface outcrops within about 5 km to the caldera fault, and a thick layer of this unit beneath the Xaltipan ignimbrite (Carrasco-Núñez et al. 2018).
Mazama. The Mazama-Crater Lake caldera was formed relatively recently in a 'cataclysmic' eruption 7.7 ka ago that represents the first caldera-forming eruption of the volcano. Mazama has a long history of >550 ka, which is exceptionally well constrained both in terms of the geochemistry of the eruptive products and eruption volumes (Bacon and Lanphere, 2006). The large range of magma diversity was already produced in the early history of the volcano. Early eruptive products of the Mazama-Crater Lake complex are comprised of rhyodacitic to dacitic lava domes dated at ~460-400 ka and termed 'pre-Mazama' to distinguish them from the later andesitic-dacitic 'Mazama stratovolcano'. Mafic eruptions of basaltic and basaltic andesite magmas occurred over the entire history of the volcanic complex and while termed mostly 'regional volcanism', vent locations of these eruptions are within <10 km from the present-day crater rim.
Santorini. Steep unconformities in the cliffs of Santorini (Aegean arc, Greece) record evidence for at least three caldera-collapse events. The earliest geological records of caldera-formation date back to ~172 ka associated with the 'Lower Pumice 2' eruption. If earlier major explosive eruptions (Lower Pumice 1, Cape Therma 1-3) have produced calderas as well is unconstrained. To be most conservative, the pre-caldera compilation for Santorini consists of the Akrotiri lavas (650-550 ka) and the Peristeria stratovolcano (550-450 ka), which comprise the earliest phase of constrained activity of the volcano. Eruptive products of these phases are rhyodacitic to basaltic in composition (Druitt et al. 1999).